This invention relates to a process for producing sulfur and hydrogen from hydrogen sulfide by subjecting hydrogen sulfide to a decomposition reaction by use of a catalyst.
Petroleum refining processes of the type using hydrogen gas such as, for example, a process for desulfurization of crude oil or heavy oil by hydrocracking and a process for desulfurization of various petroleum fractions by hydrocracking by-produce fairly large volumes of hydrogen sulfide. There is every indication that the total volume of hydrogen sulfide thus by-produced will increase year after year in consequence of the expected increase in the size of petroleum refining facilities and in the consumption of petroleum products. In the existing petroleum refining processes, the by-produced hydrogen sulfide is generally released from the reaction systems in the form of off-gas in conjunction with other inflammable gases. The off-gas is usually used as the fuel for heating furnaces, for example. If the off-gas has a high hydrogen sulfide content, the concentration of sulfur dioxide in the waste combustion gas increases and causes enviromental pollution. This necessitates separation of hydrogen sulfide from the off-gas. As concerns the separated hydrogen sulfide, the need for converting it into some other valuable substance has become pressing.
Several methods have heretofore been suggested for effective use of the by-produced hydrogen sulfide. Of these methods, the so-called Claus method or the modified Claus method is about the only one which has commercial significance at all. This method comprises the steps of separating hydrogen sulfide from the off-gas, concentrating the separated hydrogen sulfide and subsequently converting it into elementary sulfur and water through a partial oxidation treatment, as indicated by the following reaction formulas: ##EQU1##
Although in this method, the sulfur component alone is recovered, the hydrogen component which is responsible for the majority of the cost incurred in the desulfurization by hydrogenation is partially discarded finally in the form of water, making the process highly uneconomical.
U.S. Pat. No. 2,979,384 teaches a method which comprises allowing a lower sulfide of iron, nickel or cobalt to react with hydrogen sulfide to produce a higher sulfide and hydrogen gas and then subjecting the produced higher sulfide to thermal decomposition to give rise to a lower sulfide and sulfur. This method, thus, involves two reactions in deriving hydrogen and sulfur from hydrogen sulfide.
Further, U.S. Pat. No. 2,839,381 granted to R. Lee and E. Grove discloses a method which comprises causing hydrogen sulfide by-produced in the reduction with hydrogen of sulfide ore to be electrolyzed in an aqueous solution containing potassium iodide and sodium iodide so as to produce hydrogen and sulfur and recycling the produced hydrocarbon to the process of sulfide ore reduction. For practical purpose, however, this method entails complicated steps of operation.
It is also known to the art that at elevated temperatures, hydrogen sulfide is dissociated into hydrogen and sulfur as indicated by the following formula: EQU XH.sub.2 S .revreaction. XH.sub.2 + S.sub.X
(wherein, S.sub.x denotes the allotropes of gaseous sulfur such as S.sub.2, S.sub.6 and S.sub.8).
If the equilibrated dissociation is considered for X = 2, then the equilibrium constant, Kp, is 5.13 .times. 10.sup.-.sup.3 for 800.degree. K. (527.degree. C.) and 2.82 .times. 10.sup.-.sup.3 for 773.degree. K. (500.degree. C.). From this value of the equilibrium constant, the equilibrated hydrogen concentration at 500.degree. C. is calculated to be 0.4%. This means that 0.4% of hydrogen is produced by heating hydrogen sulfide to 500.degree. C. In this case, however, the dissociation of hydrogen sulfide by mere application of heat proceeds with extreme slowness so that, even after about 20 hours of heating, it will not reach the stage of equilibrated hydrogen concentration. Thus, it is substantially impossible to detect any formed hydrogen in this reaction. To render this reaction commercially feasible at all, it is necessary that the reaction velocity be notably heightened.
Molybdenum sulfide (MoS.sub.2) and tungsten sulfide (WS.sub.2) which are used for the present invention are adopted as shown below, either by themselves or in the form of multi-component catalysts (frequently supported on alumina or silica-alumina) having nickel sulfide or cobalt sulfide combined therewith. For example, they are used as catalysts in the hydrocracking of various petroleum fractions by U.S. Pat. No. 3,267,021, as catalysts in the cracking by U.S. Pat. Nos. 3,340,422 and 3,475,325 and as catalysts in the desulfurization of heavy oil, etc. None of them, however, has ever found utility in the decomposition of hydrogen sulfide, by one step, into hydrogen and elementary sulfur.
An object of this invention is to provide an improved commercially useful process for the production of hydrogen and sulfur by the decomposition of hydrogen sulfide. Another object of this invention is to provide an improved commercially useful process for the continuous production of hydrogen and sulfur by the decomposition of hydrogen sulfide.